What's a Splint?

Many affected horses regain a normal state of soundness, with perhaps only a small bump on the leg remaining as a reminder of the injury.

Photo: Paula da Silva/www.arnd.nl

There is a bit of benign confusion about the area of the horse's legs that house what most horse owners call the "splint bones." On each side of the metacarpal or metatarsal bones (cannons front and back) is a small bone that is commonly called the "splint bone." Each is attached to the cannon by an interosseous (between bone) ligament, thus the two small bones "splint" or support the large bone. (Now, probably for the first time, you know why that area got its name.) For many years, horse owners and veterinarians thought these were just remnants of toes from the prehistoric horse, but the splint bones--known as the second and fourth metacarpal (front legs) or metatarsal (hind legs) bones--provide considerable support to the cannons.

When a horse "pops a splint," it can be painful to the animal and unsightly as well. The good news is that, generally speaking, the prognosis for a return to normal activity is positive. There are times, however, when problems with these small bones and their connections to the cannon bone can interfere with normal athletic endeavors. Following, you will find information on the area in question, the common problems associated with it, and treatments to alleviate those problems.

Splint Bone and "Popped Splint" Anatomy

The splint bones start at the knee in the front legs and run downward along the cannon bones. They are larger at the upper end that begins at the knee joint than they are at the lower or posterior end that stops about three-fourths of the way down the cannon bone.

The splint bone on the inner (medial) side of the front leg is known as the second metacarpal bone, and the splint bone on the outer (lateral) side of the leg is the fourth metacarpal bone. The cannon bone is the third metacarpal bone.

The splint bones in the rear legs begin at the hock and travel downward, also coming to a stop about three-fourths of the way down the rear cannon bone (third metatarsal). The splint bones in the rear legs are the second and fourth metatarsals.

What is a "popped splint?" Technically, it is known as interosseous desmitis. Desmitis means inflammation of a ligament, and the interosseous ligament is the one that runs between the splint bone and the cannon bone. You can also have fractures or breaks in the splint bone itself that cause the "bumps" horse owners commonly call splints.

While splints can be "popped" in both front and rear legs, this is more apt to occur in front. And, it most commonly occurs in the second metacarpal--the splint bone on the inside or medial aspect of the leg. There is a reason for this. For one, there is more weight-bearing stress on the inner splint bone than the outer one, and there is more of a chance for interference if the horse has poor conformation or is improperly shod.

For example, the base-narrow, toed-out horse will wing inward with its front feet as it travels. That raises the threat of striking the opposite front leg, especially at the trot. Such a blow can cause trauma and inflammation to the splint bone. When such an injury occurs, nature makes an effort to repair the damage. Often, the repair work involves the formation of new bone material that, in essence, fuses the splint bone to the cannon bone. The new bone growth can result in a permanent bump on the horse's leg unless it is surgically removed.

More often than not, rear leg splints occur to the fourth metatarsal--the one on the outside or lateral aspect--and frequently are result from a kick by another horse.

Other types of trauma, in addition to a blow, can cause a horse to "pop" a splint. Included as culprits are poor conformation, such as bench knees, which produce undue stress; improper shoeing that produces undue strain; or a heavy training regimen with young horses, such as 2-year-olds, that is more than their legs can handle. Malnutrition can also be a contributing factor. Conversely, too much nutrition can play a role if it causes bones to develop too rapidly or the horse to gain more weight than his limbs can tolerate.

Splints are most apt to occur in young horses, but older animals aren't immune.

Types of Splint Injuries

In the textbook Adams' Lameness in Horses, fourth edition, splint conditions are broken down into four categories. Following are those categories and comments on each type. If your horse "pops" a splint, it is wise to have your veterinarian check it out. A casual approach to a splint injury might fail to detect a more serious condition, such as a fracture of the splint bone.

True Splint--This refers to a sprain or tear of the interosseous ligament. The resultant enlargement is most frequently observed just below the knee and on the inner side of the leg (medial aspect). Most horses with a true splint will experience lameness, although it is usually transient, says Mark J. Martinelli, DVM, PhD, Dipl. ACVS, Associate Surgeon (Orthopedics) at the San Luis Rey Equine Hospital in Bonsall, Calif. This problem is not uncommon, especially in young horses entering heavy training.

Blind Splint--This refers to an inflammatory process of the interosseous ligament that can be difficult to detect because it is between two bones, thus the term "blind" splint. "This is readily detectable with scintigraphy, and sometimes ultrasound can pick it up," says Martinelli, "although it is not as common as a true splint."

Knee Splint--This can be a particularly compromising form of splint because it refers to the enlargement of the upper end of the splint bone at the knee. New bone growth in this area can lead to osteoarthritis within the joint. The result is a sore, unsound horse. "This is even less common," says Martinelli. "It usually only occurs with trauma or really poor conformation like dramatically offset knees."

Periostitis of the Splint Bone--This usually results from superficial trauma to the periosteum (the sheath that covers the bone) which, in turn, causes it to become inflamed. Quite frequently splints of this type result in a permanent bump on the leg, but the horse might not be lame, or, if it does become lame, it will be so for only a short period of time.

The size of a splint bump can vary a good deal. Much depends on the degree of inflammation and the surface area involved.

When injury or trauma to a splint bone first occurs, there is apt to be pain, heat, and swelling in the affected area, followed by the formation of a bump. The initial bump that you see on the leg is in the form of fibrous tissue that nature uses to try and stabilize the area. Later, this will ossify (develop into bone). The initial bump might well be larger than the one that is present once the inflammation subsides. The reason is that not all of the fibrous tissue develops into bone.

Fracture of the Splint Bone--When a fracture occurs, the swelling and heat normally will be noticeable over a large area and the horse might remain lame for a much longer period. This is not the time for a layman to make a diagnosis--if there is a splint bone injury, a veterinarian should be involved. In some cases, the only definitive diagnosis is via radiographs.

"Splint bones usually fracture for one of two reasons--direct trauma or as the result of a suspensory ligament injury in high-performance horses," explains Martinelli. "A small ligament from the suspensory ligament attaches to the distal end of the splint bone. Once a suspensory ligament has been injured, the fetlock drops lower than normal (hyperextends) at high speed. This puts greater tension on the ligament attaching to the splint bone, sometimes fracturing the splint bone and pulling the lower part away from the upper part."

In some cases, fractured splint bones will heal on their own with the help of stall rest. However, if they don't heal in a short time, surgery might be necessary to remove the lower portion of bone.

"Surgery to remove a splint bone is a minor procedure in most cases," Martinelli says. "However, when the fracture is caused by a suspensory ligament injury, the limiting factor for return to soundness will be the health of the suspensory ligament itself."

Treatment

There are a great many viewpoints on treating splint injuries other than fractures in the veterinary field, but one that has widespread backing and appears to cover many bases is offered in Adams' Lameness in Horses.

As already stated, in the acute phase of splint problems, inflammation and swelling, are the hallmark of the condition. To combat this, Adams and collaborators suggest, phenylbutazone should be administered and cold packs should be applied to the affected area. After treatment, a pressure support wrap is recommended.

When applying cold therapy either with ice packs or whirlpool boots, it is recommended that it be continued for 30-45 minutes twice a day for at least five to seven days. It has also been recommended by some veterinarians that the affected area be massaged by hand for 10 minutes after each cold pack application and that a support bandage then be applied.

After 10 days, when the inflammation is gone, a mild liniment can be applied underneath the support wraps, according to the text.

Still another recommendation is that affected horses should be confined to a stall for at least 30-45 days and hand walked for 15-20 minutes twice a day after the acute inflammation subsides.

There, of course, are other approaches that your veterinarian might suggest, and they likely will vary horse by horse and injury by injury. In some cases, surgical removal is the only recourse if the horse is to become sound. For example, if the new bone growth impinges on movement of the suspensory ligament, the horse might be lame permanently unless the bone growth is removed.

Can splints be prevented? The answer is yes in a number of cases. For example, if a horse has improper conformation--such as base narrow, toed out--protective boots should be used whenever the horse is ridden to prevent one front limb from causing damage if it strikes the other. If improper shoeing causes inappropriate hoof flight, that should be rectified immediately.

Eliminating heavy exercise in young horses can also help. Conditioning them via a program that gradually intensifies the workload can serve as a strong preventive measure by adapting the muscles, bones, and ligaments instead of overstressing them. Included along the way should be periods of rest.

Finally, when a horse develops a splint, it should not be ignored. Veterinary advice should be sought and followed. If it is, the chances are excellent that the horse will regain a normal state of soundness, with perhaps only a small bump on the leg remaining as a reminder of the injury.

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